The present invention generally relates to an apparatus and method for controlling vehicle motion. More specifically, the invention relates to an apparatus for improving vehicle stability by controlling the brake torque of a vehicle during, for example, cornering.
During vehicle motion such as cornering, both longitudinal forces (i.e., front to back) and lateral forces (i.e., side to side) influence the lateral and longitudinal behavior of the vehicle, as noted in the article xe2x80x9cA Study On Vehicle Turning Behavior in Acceleration and in Brakingxe2x80x9d, SAE Technical Paper No. 852184, pages 75-86, by Masato Abe which is hereby incorporated by reference. As further noted in the article, complicated equations of motion are involved in describing the combined lateral and longitudinal behavior of the vehicle, because many of the steady state equilibrium conditions which may exist during a constant speed mode of operation might not exist during vehicle braking or acceleration.
The varying longitudinal forces which affect vehicle stability during braking or acceleration have a tendency to cause the rear wheels of a vehicle to lock during braking due to a varying decrease in the rear wheel load. In order to prevent this rear wheel lock from occurring, some prior art brake control systems include a proportioning valve to adjust the amount of braking in proportion to the longitudinally changing loads of the front of the vehicle relative to the back of the vehicle.
Although the use of such a proportioning valve helps to prevent rear wheel lock from occurring during braking due to longitudinally changing load forces, it does not sufficiently adjust the braking action at the vehicle wheels to compensate for vehicle load changes that are due to lateral, i.e., side to side, forces. When a vehicle is undergoing a cornering maneuver, for example, there is not only a longitudinal load shift in a tangential direction to the vehicle""s path of motion, but there is also a lateral load shift in a direction which is normal to the vehicle""s path of motion. Such a lateral load shift is transferred, for example, from the wheels located on the inside of the curve in the vehicle""s path to the wheels located on the outside of the curve in the vehicle""s path. It is this lateral load shift which urges the vehicle out of its current path as defined by an existing radius of curvature, and into an oversteer or an understeer condition.
In the aforementioned article by Masato Abe, a study of the affect of acceleration and braking on vehicle turning behavior is presented. In this study, equilibrium equations of vehicle motion for constant lateral and longitudinal accelerations which describe the vehicle turning behavior during acceleration and braking are developed. The equations derived are used to obtain the radii of curvature of the vehicle path versus vehicle forward speed during constant acceleration or braking in turns. The vehicle turning behavior is also described by a characteristic line representing the lateral acceleration versus the longitudinal acceleration for a circular turning maneuver. For example, FIGS. 5-7 of the article reflect that for a given steering wheel angle, increased deceleration due to, for example, braking action (as reflected by negative acceleration in the FIGS. 5-7), results in a change from an understeer condition (i.e., an increase in turning radius), to an increasingly severe oversteer condition (i.e., a decrease in turning radius), with increased vehicle speed.
Although the prior art has recognized that longitudinal forces as well as lateral forces affect the vehicle motion during cornering, there is a need to provide a vehicle motion control system which will actually compensate for the lateral forces that detrimentally influence vehicle stability during the course of vehicle motion.
Accordingly, it is an object of the present invention to overcome the deficiencies of the prior art by providing a novel apparatus and method for improving vehicle stability. In accordance with the invention, vehicle motion is controlled through the use of a brake controlling system which compensates for the influence of lateral forces on the vehicle.
More specifically, an apparatus for controlling vehicle motion is provided which independently controls braking torque in response to a sensed actual yaw rate. In a preferred embodiment, the apparatus for controlling motion includes a means for measuring the actual yaw rate of the vehicle. The apparatus also includes a means for determining a desired yaw rate of the vehicle and for producing an output signal in response to a comparison of the desired yaw rate with the actual yaw rate. The desired yaw rate is determined on the basis of the vehicle""s steering angle and velocity. Accordingly, a first sensor means is provided for detecting the steering angle at which the vehicle is turning, and a second sensor means is provided for detecting vehicle speed. The second sensor means includes a plurality of wheel speed sensors for detecting the speed of rotation of each vehicle wheel independently so that an accurate indication of vehicle speed can be obtained from the average of the wheel speed sensor outputs.
The output signal produced in response to the comparison of a desired yaw rate with an actual yaw rate is applied to a braking control means. Based on this output signal, the braking control means maintains the handling characteristics of the vehicle neutral (i.e., prevents oversteer or understeer) or, at the most, permits only negligible understeer to occur during a maneuver such as cornering.
More specifically, if there is a discrepancy between the measured yaw rate and the desired yaw rate, the existence of lateral forces which could detrimentally influence vehicle motion is indicated. If the measured yaw rate is determined to be less than the desired yaw rate, the brake controlling means will increase the brake force applied to the vehicle""s wheels which face the inside of a curve in a vehicle""s path and/or decrease the brake force applied to the vehicle""s wheels which face the outside of the curve. On the other hand, if the measured yaw rate is greater than the desired yaw rate, the brake controlling means will decrease the brake force applied to the inside wheels of the vehicle and/or increase the brake force applied to the outside wheels. However, if the output signal indicates that the actual yaw rate and the desired yaw rate are equal, then no action is taken.